Well the tuits finally reached the required level. Also the forecast advises the boiler won't be missed for a few days.
Progress and concerns so far:-
A benefit of underfloor heating manifolds is they are fitted with isolation valves so I have only to drain/re-fill the primary piping. Water came out gin clear.
The inbuilt pressure vessel was holding gas but down to 7.5psi. Also a teaspoon of water came out (Valve at bottom) when I fitted the pump to re-pressurise. Back to 1 bar now.
There has been a modest leak on the ground floor manifold valvery. Not certain which joint as they are well larded with limescale. The thermal regulator most likely. Conveniently that part of the system can be isolated so I could have another go if it persists beyond the 1/4 turn already given.
Next job is to re-route the relief valve to the pumped side of the diverter valve! I may be gone some time..